352 Annual Report for 1913 of Royal Veterinary College. 
manufacture it for a considerable and often long period after 
recovery. When the bacteria of the same disease' are introduced 
into the body of such a recovered animal they are acted upon 
directly or indirectly by this substance, in consequence of 
which their multiplication is prevented or checked and the 
animal escapes a second attack. 
When a recovered animal has such an immunising substance 
in its blood it would be natural to expect that its blood if trans- 
ferred in sufficient quantity to another animal ought to 
immunise the latter, and in some cases that is found to be the 
actual fact. In general, however, one cannot usefully employ 
the blood of a recovered animal for this purpose because the 
quantity necessary to immunise another animal would be 
too great. The recovered animal’s blood is, so to speak, too 
weak in the immunising substance ; but fortunately it can be 
greatly strengthened by a process which is termed hyper- 
immunising. This consists in inoculating the animal at intervals 
with gradually increasing doses of the bacteria which are the 
cause of the disease, and under this stimulus increasing 
quantities of the immunising substance are manufactured and 
accumulate in the blood. Hence, the blood of such a hyper- 
immunised animal may even in a small dose suffice to protect 
another animal into which it is injected. 
The preparation of an anti-swine fever serum is attended 
with special difficulties, (1) because, as the pig is the only 
animal susceptible to swine fever, other animals, such as horses 
or cattle, cannot be used as serum producers ; and (2) because, 
since the swine fever organism will not grow outside the body, 
one cannot employ artificial cultures for hyper-immunising, but 
must rely for this purpose on the blood of pigs suffering from 
swine fever. 
The manufacture of the serum is carried on in the following 
way. Large numbers of healthy pigs are infected with swine 
fever, and at the height of the disease they are bled to death, 
the blood being collected in a sterile condition. This highly 
virulent blood is used for hyper-immunising selected healthy 
pigs which are to serve afterwards as the serum-producers. 
The animals of this set must already have some immunity in 
consequence of their having recovered from an attack of swine 
fever, or thej must be given a dose of anti-swine fever serum. 
This is necessary because it is evident that otherwise the first 
dose of virulent blood given to them would probably set up a 
fatal attack of swine fever. 
After repeated injections of virulent blood at fortnightly 
intervals, the serum-producing pigs are themselves bled, and it 
is the serum obtained from their blood which is used in practice 
to treat the animals where swine fever has broken out. 
